This invention relates to temperature and acid-resistant flexible hollow couplings, sometimes referred to as expansion joints. Expansion joints connect pipes, ducts and the like, and such joints are usually installed to allow limited movement of one part relative to another due to thermal expansion, vibration, misalignments and the like. Such joints are typically used to connect fluid inlet or outlet ducts, such as are found in large drying, exhaust, heating, ventilating and power generating systems. These flexible joints may be quite large and may be made of a continuous length or belt of flexible material sometimes having a flange along each edge for connection to corresponding flanges of the ductwork.
Typical expansion joints are illustrated in U.S. Pat. No. 3,460,856, U.S. Pat. No. 3,633,946, U.S. Pat. No. 3,874,711, U.S. Pat. No. 3,997,194, U.S. Pat. No. 3,934,905, U.S. Pat. No. 3,647,247, and U.S. Pat. No. 3,811,714.
Conventional expansion joints are usually made from a coated heat resistant fabric that is also impervious to prevent leakage of gas therethrough. Multiple layers of materials may be required to provide satisfactory insulating properties in a hot environment. Conventional materials have included elastomers vulcanized onto woven fabrics of inorganic fibers such as asbestos or fiberglass, or organic fibers, such as aramids. In some cases, metal wire is inserted into the yarns before weaving for improved properties. Woven fabrics, however, may not be stretched to any significant degree, i.e., typically less than 20 percent, and it is necessary to use sufficiently excess material in the joint to accommodate any forceable movement between the adjacent ducts.
Difficulties have been experienced with the durability of expansion joints wherein the joint is composed of a laminate of rubber and internal reinforcing woven materials, such as those composed of glass, asbestos, or synthetic polymer materials. Many applications for expansion joints require continuous operation at temperatures above 300 to 400 degrees F and short durations to 800 degrees F wherein the joint may also be subjected to a highly corrosive atmosphere, such as acid vapor or gas. Under these circumstances, such conventional joints tend to rather quickly degrade, losing tensile strength and flexibility, and the elastomer layers tend to delaminate from the fabric layer.
Fluoroelastomers generally have higher temperature ratings than other elastomers, and it would be desirable to use such elastomers in expansion joints. Unfortunately, however, various problems arise when fluoroelastomers are used in combination with reinforcing fabrics composed of materials such as asbestos, glass or synthetic polymer textiles. Fluoroelastomers can emit corrosive acids at elevated temperatures. In some cases, it is difficult to form a strong bond in the lamination process. Under actual service conditions, the fibers tend to absorb acid vapors, thus accelerating degradation of the joint. Also, it has been found that combinations of glass fibers and fluoroelastomers are unstable at the higher temperatures, with the result that the elastomer loses flexibility and adhesion to the fibers.